1. Field of the Invention
This invention relates to a method and apparatus for treating glass sheet edges and particularly to the flame treatment of the peripheral edges of glass sheets.
2. Technical Problems and the Prior Art
Flat glass is cut to desired outline configurations by drawing a cutting tool over one major surface of the glass to score the surface along a line conforming to the outline configuration desired. The cut is then completed by applying pressure to the glass on opposite sides of the score line while supporting the glass sheet on its other major surface along a line directly below the score line. During scoring with a cutting tool, the surface that is initially breached often develops vents emanating from the score line. These vents are flaws in the scored surface such as hair lines, chips or other flaws that may open up to cause the glass to rupture, crack or shatter when subjected to subsequent heating and cooling required to bend and/or temper the glass.
It has been known for some time that it is important to remove the vents and treat the glass in such a manner that the peripheral edge of the glass is smooth and free of vents, particularly when the cut glass sheet is to be bent and tempered to form automobile back lights. Rough edges have been smoothed somewhat by sanding or otherwise abrading the rough cut edge. However, mechanical operations of this type are slow and, unless the edges are subjected to prolonged abrasion, vents remain that cause later rupture of the glass.
Flame seaming has been developed to strengthen either a straight edge or the entire perimeter of a glass sheet cut to an irregular outline. An open flame jet comprising a single elongated jet or a series of closely spaced jets impinges against an edge to be seamed. In case of a straight edge, the straight edge is moved linearly past an open flame jet comprising a series of closely spaced jets directed against the center of said straight edge so that each increment along said straight edge is subjected to successive impingement by all of said closely spaced jets. Tumblers having rounded peripheries have their peripheral rims fire finished either by exposure to an annular burner or by rotating said tumblers to pass each increment of the rim in close relation to a series of closely spaced burners mounted in an arcuate arrangement to heat each increment of the rim as the tumbler rotates while going from a loading station to an unloading station in an arcuate path past the arcuate arrangement of burners.
Glass sheets cut to irregular outlines have been flame seamed by supporting a glass sheet precut to irregular outline in fixed relation within a flame seaming apparatus spaced outwardly from said glass sheet outline and constructed and arranged to impinge an open flame jet against the edge to be seamed to simultaneously fire polish the complete edge of the sheet. The open flame jet originates either from a pipe having a plurality of closely spaced ports facing the entire edge of the glass sheet so that jets of burning gas combine to provide a continuous sheet of flame impinging against the glass sheet edge or from a pipe having a slot-type continuous orifice. While the purpose of this system is to provide a uniform heat flux to the entire edge of the glass sheet by directing a sheet of flame in a direction parallel to the major surfaces of the glass sheet to impinge against the center of the glass sheet edge, small variations in the burner flame front occur. As a result of these small variations, small irregularities develop in the regions of the fire polished edge that are subjected to deviations from uniformity of the impinging flame front. While translating a straight edge in a straight line direction relative to a straight source of burning gas compensates for irregularities in the fire polish finish of a straight edge and a relative rotational movement of a round edge surface relative to an accurately constructed burner compensates for irregularities in the fire polished finish of a rounded edge, neither of these techniques are suitable to improve the uniformity of the fire polished finish of a glass sheet having an irregular outline that includes both straight and curved portions.
U.S. Pat. Nos. 2,971,294 and 3,008,270 to Hohmann et al. teach a method and apparatus for flame seaming either the entire edge periphery or one straight edge only of a precut glass sheet of irregular outline. A flame seaming burner comprising a pipe member corresponding in shape to the outer irregular peripheral configuration of the piece of glass and having a series of apertures formed through its inner periphery is aligned with the glass sheet edge so that flame jets of a suitable gas and air mixture are directed against the center of the edge of the irregularly shaped glass sheet. Flame seaming is achieved in these patents when the flame burner apertures are No. 50 size drill holes arranged on 1/4 inch centers with the mouths of the jets approximately 1 inch from the edge of the glass sheet to be treated. There is no relative motion between the glass sheet and the pipe member for a period of 10 to 15 seconds while the glass sheet is exposed to the burning gas jets. In the embodiment to flame seam a straight edge, the glass sheet is passed through a series of closely spaced flame jets applied to the straight edge. No solution is suggested to reduce any irregularity that may develop in the fire polished edge of a sheet of irregular outline. The second embodiment, which teaches relative linear motion of a straight glass sheet edge relative to a straight line series of flame jets, is not appropriate for any other shape of edge other than straight edge.
U.S. Pat. No. 3,592,620 to Carlisi et al. teaches introducing certain thermally decomposable compounds into a fire polishing flame applied to an edge of a glass laminate to cover an edge gap between layers that are adhered at the major surfaces of a glass sheet laminate, but leave an edge portion exposed to form said edge gap.
U.S Pat. No. 2,883,797 to Eldred and U.S. Pat. No. 3,107,161 to Bivens et al. relate to techniques for fire polishing the circular rims of glass tumblers. Eldred rotates the edge through a series of jets of burning gas arranged along an arcuate path to heat the rim in a uniform manner. Bivens et al. exposes the entire circular rim to a ring burner for a short duration of 1 to 2 seconds to fire finish the surface of the rim without materially causing an out-of-round condition in the rim. These last two patents are not capable of flame seaming a glass sheet having an outline configuration that is irregular rather than round.